Llanite

Llanite is a porphyritic rhyolite with distinctive phenocrysts of blue quartz (a rare quartz color) and perthitic feldspar (light grayish-orangish). The brown, fine-grained groundmass consists of very small quartz, feldspar, and biotite mica crystals.

Llanite
Llanite from the dike on Texas State Highway 16

Llanite comes from a hypabyssal porphyritic rhyolite dike that intrudes Precambrian metamorphics in the Llano Uplift of central Texas. Published radiometric dating on this llanite indicates that it's 1.106 billion years old (late Mesoproterozoic).

The quartz crystals found in llanite are blue hexagonal bipyramids. The unusual blue coloration of the quartz is thought to be due to ilmenite inclusions.

It is named after Llano County, Texas, the only place where it is found.[1]

However, the geology of North-East Africa is very similar to that of Texas. Many of the minerals and fossils found are only found in these two locations on the whole planet. This is because when the world was the supercontinent Pangaea, Africa's coast joined with Texas'coast.

(geologists have identified other locations where similar types of rock may be found).[2] A dike of llanite crops out on Texas State Highway 16 about 9 miles north of the town of Llano.[3]

Llanite, which is similar to granite, is very strong, with a crushing strength of 37,800 lb/in2 or 26,577,180 kg/m2.[4]

Modal mineralogy of llanite mode:[5]

  • quartz - 34.6%
  • microcline - 27.8%
  • plagioclase - 27.9%
  • biotite - 8.6%
  • fluorite - 1.1%
  • apatite - 0.14%
  • magnetite - trace
  • ilmenite - trace
  • zircon - trace

See also

References
  1. Zolensky, M.E., Sylvester, P.J., and Paces, J.B. (1988) Origin and significance of blue coloration in quartz from Llano rhyolite (llanite), north-central Llano County, Texas. Am. Min. v. 73, p. 313-323.
  2. Reed, R. nd, Llanite in Africa. Archived 2012-03-19 at the Wayback MachineLlanite, Archived 2014-04-28 at the Wayback Machine Rob's Granite Page Archived 2011-04-04 at the Wayback Machine, The University of Texas at Austin, Austin, Texas.
  3. Roadside Geology of Texas, 1991, by Darwin Spearing
  4. Barnes, Virgil E. (1988) Geological Society of America Centennial Field Guide-- South-Central Section, p. 361-368.
  5. Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyro liparose-alaskose) from Llano, Texas. Journal of Geology, 12, p. 225-231.

Heinrich, P. V., 2014, Llanite and the Blue Quartz of Texas. The Backbender's Gazette. vol. XLV, no. 5, pp. 5–12. (Houston Gem and Mineral Society, Houston, Texas).


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